Synthesized Four Channel Stereo From A Two Channel Source

Abstract

This invention relates to a synthesized four channel output from a two channel stereo source. The invention operates on left and right ((L), (R) input signals to produce difference signals (L-R), (R-L) by combining a left signal with a phase shifted right signal and by combining a right signal with a phase shifted left signal. The amount of phase shift introduced on each phase shifted signal is frequency dependent and varies from 0.degree.-180.degree. with low frequency components being 180.degree. out of phase and the high frequency components substantially in phase. In addition another phase shift is introduced on at least one difference signal, to place a phase shift of less than 90.degree. between the two difference signals. The two difference signals are then fed to the rear speakers in a four channel reproduction system.

Description

This invention relates to a multidimensional stereophonic reproducing system and more particularly to an artificial multidimensional stereophonic reproducing system such as an artificial four channel system.

There has been used a multidimensional stereophonic reproducing system in which information signals obtained from various sound sources distributed in a space are recorded or transmitted as multichannel signals and the multi-channel signals are reproduced by a plurality of reproduced sound sources on the reproduction side thereby to give a stereophonic impression to a listener. As a two dimensional stereophonic reproducing system, there has been used a two channel stereophonic reproducing system in which two channel signals, i.e., left channel and right channel signals, are reproduced from speakers arranged at left front and right front of the listener. Besides this two channel system, there have been proposed and used four channel stereophonic reproducing system such as a front four channel stereophonic reproducing system in which four channel reproduced sound sources are all arranged in the front of the listener (hereinafter referred to as "4-0" system), a two front channel-two rear channel stereophonic reproducing system in which two channels of reproduced sound sources are respectively arranged in the front and in the rear of the listener (hereinafter referred to as "2--2" system) and a three front channel-one rear channel stereophonic reproducing system in which three channels of reproduced sound sources are arranged in the front of the listener and one channel of reproduced sound source is disposed in the rear (hereinafter referred to as "3-1" system).

It has been recognized that these four channel stereophonic reproducing system is by far superior to the conventional two channel stereophonic reproducing system in respect of degree of reality, clearness in sound source orientation etc. It is to be noted, however, that a multidimensional stereophonic reproducing system of a multi-channel type such as four channels requires multi-channel programme sources such as four channels.

This invention is designed to provide a multidimensional stereophonic reproducing system in which a stereophonic reproduced sound field according to the 2--2 system or the 3-1 system can be artificially formed by employing two channel programme sources used in a conventional stereophonic reproducing system.

In the aforementioned 2--2 system and 3-1 system, the sounds radiated from the reproduced sound sources disposed in the rear of the listener are mainly reflected sound, reverberation of the hall etc. It is conceivable, therefore, to obtain a signal which resembles the rear channel signal in the 2--2 system or 3-1 system through a delay reverberation apparatus and utilize it as an effect sound for an artificial multidimensional stereophonic reproduction. However, a delay reverberation apparatus which is not of a large type used for business purposes but of a simple one which can be assembled in an apparatus for a domestic use is generally insufficient in its performance and is short in its life. Besides, even this simple type delay reverberation apparatus is considerably expensive so that the manufacturing cost of the reproducing apparatus becomes high. Consequently, the system which employs the aforementioned delay reverberation apparatus cannot be put to a practical use due to the aforementioned disadvantages.

The present invention provides a system in which a direct sound component (e.g., a direct sound component of a musical instrument or human voice) from a signal source which constitutes a two channel stereo signal is effectively cancelled or reduced and an indirect sound component including reflected sound of a musical instrument or human voice, reverberation of the hall and a delay reverberation component which is artificially added during mixing operation in recording is effectively obtained and utilized. A direct sound from a sound source generally reaches both left and right microphones with an equal sound pressure and phase. However, sound from the sound source which reaches the two microphones after irregular reflection from walls, floor, ceiling etc. has little or no regular characteristics which the direct sound has because of its irregular path of travel. This irregular reflected sound contributes to forming a vivid feeling of reality. In the system according to the present invention, a difference signal between the left channel signal and the right channel signal which compose the two channel stereo signal is reproduced from rear speakers thereby to sound the indirect sound.

In case the direct sound component of the two channel signals L and R contain various components which are different in level or phase from each other, the direct sound component tends to be included in the effect sound signal for an artificial multidimensional stereophonic reproduction. This problem can be overcome, as will be described later, by suitably adjusting the level and phase of the direct sound component of each channel signal L and R before the difference signal is obtained from the channel signals L and R.

Again, if a low frequency component is lacking in the direct sound component reproduced from the rear speakers, the degree of reality is decreased and the indirect sound forms a reproduced sound field which gives a loose, scattered impression thereby causing unpleasant-ness, feeling of disharmony or phychological instability to the listener. The system according to the invention, therefore, is designed to settle this problem. According to the invention, a modified signal which is obtained by inverting (changing) the phase of a low frequency component of each channel signal is used for obtaining a difference signal, so that a low frequency component is not lacking in the indirect sound component of the difference signals.

It is, therefore, a general object of the invention to provide a novel and useful system in which a plurality of channel signals are used for a stereophonic reproduction which is artificially made more multi-dimensional than the actual number of channels.

Another object of the invention is to provide a system in which indirect sounds such as reflected sound and reverberating sound are obtained from difference signals of a plurality of channel signals and a multi-dimensional stereophonic reproduced sound field is formed by reproducing and sounding the indirect sounds from side or behind the listener.

A further object of the invention is to provide a reproducing system in which a direct sound component is almost completely cancelled and an excellent difference signal component is obtained. According to the system, the direct sound component is almost completely cancelled by using a level correction circuit and a phase correction circuit.

A still further object of the invention is to provide a reproducing system in which difference signals are obtained by crosstalking the plurality of channel signals with a suitable amount of crosstalk in an opposite phase. By obtaining difference signals having a low sound frequency component, a multidimensional stereophonic reproduced sound field which has a vivid feeling of reality and does not cause unpleasantness or feeling of disharmony to the listener is formed.

Other objects and features of the invention will become apparent from the description made hereinbelow with reference to the accompanying drawings, in which:

FIG. 1 is a block circuit diagram for explaining the principle of first embodiment of the reproducing system according to the invention;

FIG. 2 is a block diagram of a second embodiment of the reproducing system according to the invention;

FIG. 3 is a circuit diagram of one embodiment of electrical circuit of the essential part of the block diagram shown in FIG.2;

FIG. 4 is a graphical diagram showing a phase correction characteristic curve of a phase correction circuit shown in FIG. 3;

FIG. 5 is a block diagram of a third embodiment of the reproducing apparatus according to the invention;

FIG. 6 is a circuit diagram of one embodiment of an electrical circuit of the essential part of the block diagram shown in FIG.5;

FIG. 7 is a graphical diagram showing a phase difference characteristic between two channel signals of a channel signal phase shifting circuit;

FIG. 8 is a circuit diagram showing one concrete modified example of the electrical circuit of the essential part of the circuit shown in FIG.6;

FIG. 9 is a graphical diagram showing a frequency-phase characteristic of the phase shifting circuit shown in FIG.8;

FIG. 10 is an electrical circuit diagram of a fourth embodiment of the reproducing apparatus according to the invention; and

FIG. 11 is a diagram showing a frequency-phase shift characteristic of the phase shifting circuit shown in FIG.10.

FIG. 1 is a block circuit diagram of a theoretical first embodiment of the system according to the invention. A left channel signal L and a right channel signal R respectively applied to input terminals 11 and 12 pass through preamplifiers 13 and 14 and are amplified in output amplifiers 15 and 16. The signals L and R thus amplified are respectively supplied to a speaker 18 disposed in the left front and a speaker 19 disposed in the right front of a listener 17 and sounded from these speakers.

In the meanwhile, the signals L and R which have passed through the preamplifiers 13 and 14 are respectively applied to the bases of transistors 21 and 22 of a matrix circuit 20 shown by a broken line. A resistor 24 connected to the emitter of the transistor 21 and a resistor 25 connected to the collector of the transistor 22 are commonly connected to an amplifier 27. A resistor 23 connected to the collector of the transistor 21 and a resistor 26 connected to the emitter of the transistor 22 are commonly connected to an amplifier 28. Signals +L and -L are respectively output from the emitter and collector of the transistor 21, whereas signals +R and -R are respectively output from the emitter and collector of the transistor 22. Accordingly, a difference signal (L-R) is supplied to the amplifier 27, and a difference signal (R-L) is supplied to the amplifier 28. The signals (L-R) and (R-L) amplified by the amplifiers 27 and 28 are respectively supplied to a speaker 29 disposed in the left rear and a speaker 30 disposed in the right rear of the listener 17, and are reproduced and sounded from these speakers.

Now these difference signals (L-R) and (R-L) will be considered more in detail. In these difference signals, the component (direct sound component) having the same level and phase in the L and R signals have been cancelled by each other and do not exist now. Consequently, the direct sound from the sound source is not included in these difference signals, but only the indirect sound compoment such as a reflected sound and reverberation in the hall is contained.

Accordingly, only the indirect sound such as the reflected sound and the reverberating sound are sounded from the speakers 29 and 30 relative to the listener 17 as an effective sound, and a four channel stereophonic reproduction is artificially made on the basis of a two channel programme source. Therefore, a stereophonic sound field which has a greater degree of reality than the sound field in the two chennel reproduction system using only the frong speakers 18 and 19 is formed.

There still remains a problem that in case the two channel signals L and R contain various components which are different in level and phase from each other, the direct sound component is included in the difference signals (L-R) and (R-L). An embodiment in which this problem has been overcome will be described with reference to FIGS.2 to 4.

FIG.2 is a block diagram of the second embodiment of the system according to the invention. In FIGS.1 and 2, the same component parts are designated by the same reference numerals, and the description thereof will be omitted. A left channel signal L and a right channel signal R applied to the input terminals 11 and 12 pass through the preamplifiers 13 and 14 are supplied to the left front speaker 18 and the right front speaker 19 through the amplifiers 15 and 16 as in the above described first embodiment, and are reproduced and sounded from these speakers.

The left and right channel signals L and R are on the other hand supplied to a level correction circuit 40 and corrected in level thereat. Signals L' and R' which have been corrected in level are taken out from the level correction circuit. An output signal L' from the level correction circuit 40 is supplied to a phase correction circuit 41 where the signal is corrected in its phase as will be described later. The signal L'.theta. which has been corrected in phase is then supplied to a matrix circuit 42. An output signal R from the level correction circuit 40 is also supplied to the matrix circuit 42. The signals L'.theta. and R' are matrixed in the matrix circuit 42 and output signals (L'.theta.-R') and (R'-L'.theta.) from the matrix circuit 42 are amplified in amplifiers 27 and 28. The signals thus amplified are reproduced and sounded from rear speakers 29 and 30.

FIG.3 is a circuit diagram of one embodiment of a concrete electrical circuit of the essential part in the block diagram shown in FIG.2. The level correction circuit 40 consists of two channel balancing volume controllers 50 and 51 which are actuated together. By adjusting sliders which are actuated together of the volume controller 50 and 51, the direct sound compoments in the left and right channels L and R are adjusted so as to be equal in their level.

The signal L' corrected in its level which has been obtained from the slider of the volume controller 50 is supplied through a coupling capacitor 53 to the base of a transistor 52 of the phase correction circuit 41. A phase changing circuit composed of a variable resistor 54 and a capacitor 55 is connected between the collector and emitter of the transistor 52. Base biasing resistors 56 and 57 are connected to the base of the transistor 52, and a collector resistor 58 and an emitter resistor 59 are respectively connected to the collector and the emitter of the transistor 52. It is possible to change the phase .theta. of the signal L' as shown in FIG.4 by varying the value of resistance of the resistor 54. If the value of resistance of the resistor 54 is changed from Ra to Rd in such a manner as Ra>Rb>Rc>Rd, phase characteristics corresponding to these values Ra to Rd undergo change as shown by curves Pa, Pb, Pc and Pd in FIG.4. Accordingly, the output signal of the phase correction circuit 41 which is obtained from the connecting point of the variable resistor 54 and the capacitor 55 is supplied through a coupling capacitor 61 to the base of a transistor 60 of the matrix circuit 42 as a signal L'.theta. which is the left channel signal L corrected in its level and phase.

Transistors 60 and 62 of the matrix circuit 42 are respectively connected at their emitter and collector with emitter resistors 63 and 64 and collector resistors 65 and 66. Base bias resistors 67, 68, 69, 70 are connected to the base of the transistors 60 and 62. A capacitor 71 and a resistor 72 are connected in series to the collector of the transistor 60, and a capacitor 73 and a resistor 74 are connected in series to the emitter thereof. A connecting point 80 of the resistors 74 and 76 is connected to the amplifier 27. A connecting point 81 of the resistors 72 and 78 is connected to the amplifier 28. The output signal R' from the slider of the volume controller 51 is directly supplied to the base of the transistor 62 through a coupling capacitor 79.

From the emitter and collector of the transistor 60, there are respectively obtained signals L'.theta. and -L'.theta.. From the emitter and collector of the transistor 62, there are respectively obtained signals R' and -R'. Consequently, these signals are matrixed through the resistors 72, 74, 76 and 78, and a difference signal (L'.theta.-R') is obtained from the connecting point 80, whereas a difference signal (R'-L'.theta.) is obtained from the connecting point 81. These difference signals (L'.theta.-R') and (R'-L'.theta.) are respectively sounded from the rear speakers 29 and 30.

According to the system of this embodiment, the levels and phases of the left and right channel signals L and R are suitably adjusted so that the difference signals do not contain the direct sound compoment but contain only the indirect sound compoment.

The level correction circuit 40 and the phase correction circuit 41 can be used as if they were an effect volume in a so-called delay reverberation effect apparatus to obtain a necessary output.

A more detailed examination of the difference signals in the first embodiment shows that the degree of reality is insufficient due to lack of a lower frequency compoment in the signals. In this case, the indirect sound forms a reproduced sound field which gives a loose, scattered feeling. This reproduced sound field is likely to cause a slight unpleasantness or feeling of disharmony or a phychlogical instability to the listener. A reflected sound or a reverberating sound which reaches the ear of the listener in a real concert hall includes sound of a low frequency component having a large energy. Also, the indirect sound component in the two channel signals L and R includes a large amount of low sound frequency component. In most cases, a low sound frequency component is generally included in the left and right channel signals with the same phase and level. Accordingly, if the difference signal is composed at the same level, the difference signal is devoid of a major portion of the low sound frequency compoment. An embodiment in which the foregoing problem has been overcome will be described hereinbelow.

FIG.5 is a block diagram showing a third embodiment of the system according to the invention. In FIGS.1 and 5, the same component parts are designated by the same reference numerals and the description thereof will be omitted. Left and right channel signals L and R are input from the terminals 11 and 12 and pass through the preamplifiers 13 and 14. Then the signals are supplied, on one hand, to a variable crosstalk amount adjusting circuit 90. The two signals L and R are adjusted, as will be described later, in their crosstalk amount of the same phase in the variable crosstalk amount asjusting circuit 90 and converted into signals +Li and +Ri . The signals +Li and +Ri pass through amplifiers 15 and 16, where they are converted into signals -Li and -Ri, and are reproduced and sounded from speakers 18 and 19.

The left and right channel signals L and R from the preamplifiers 13 and 14 are respectively supplied, on the other hand, to phase shifting circuits 91 and 92. Parts of the two signals L and R are respectively inverted (changed) in their phases corresponding to low sound frequency portions only in the phase shifting circuits 91 and 92, whereby the signals become modified signals +L.DELTA..theta. and +R.DELTA..theta.. Signals -L and +L.DELTA..theta. are supplied from the phase shifting circuit 91 to a matrix circuit 93. Signals -R and +R.DELTA..theta. are supplied from the phase shifting circuit 92 to the matrix circuit 93.

Difference signals -(L'-R'.DELTA..theta.) and -(R'-L'.DELTA..theta.) which are adjusted in level and matrixed in the matrix circuit 93 are respectively supplied to channel signal phase shifting circuits 94 and 95 where they are given a phase difference .phi. within 90.degree. between them. Output signals -[(L'-R'.DELTA..theta.).DELTA..phi..sub.1 ] and -[(R'-L'.DELTA..theta.).DELTA..phi..sub.2 ] of the channel signal phase shifting circuits 94 and 95 respectively pass through amplifying and frequency characteristic compensation circuits 96 and 97 and are reproduced and sounded from rear speakers 29 and 30.

Even in case the input signals applied to the terminals 11 and 12 are monaural signals, a multidimensional stereophonic reproduced sound field can be artificially formed because a low frequency portion of the signals is reproduced as rear signals.

FIG.6 is a circuit diagram of one embodiment of concrete electrical circuit which constitutes the essential part of the block diagram shown in FIG.5. The variable crosstalk amount adjusting circuit 90 adjusts the amount of crosstalk between the left and right channels L and R by a variable resistor 100, so that the separation between the two channels can be varied, for example, within a range between about 8 dB and about 20 dB. By this variable adjustment, it is possible to achieve an effect that the orientation of the reproduced sound source is shifted in the forward and backward directions of the reproduced sound field. The left and right channels L and R are crosstalked in phase in the variable crosstalk amount adjusting circuit 90 and converted to signals Li and Ri having modified information contents. The signals Li and Ri are supplied to amplifiers 15 and 16 consisting respectively of transistors 103 and 104 of a grounded emitter type. Output signals -Li and -Ri from the amplifiers 15 and 16 are reproduced and sounded from the front speakers 18 and 19 as previously described. In the foregoing and following descriptions, positive and negative symbols affixed to the signals indicate the polarities of the signals. It will therefore be readily understood that if a circuit having a construction different from the one shown in FIG.6 is used, the polarities of the signals will be different from those described above.

In the meanwhile, the left and right channel signals L and R are respectively supplied to the bases of transistors 105 and 106 through capacitors 107 and 108. Signals -L and -R are respectively obtained across collector resistors 109 and 110 of the transistors 105 and 106, and signals +L and +R are obtained across emitter resistors 111 and 112 of the same. The signals -L and -R obtained from the collectors of the transistors 105 and 106 are respectively supplied to resistors 119 and 122 of the matrix circuit 93 through capacitors 113 and 114. The signals +L and +R obtained from the emitters of the transistors 105 and 106 are respectively inverted in their phase in low frequency portions through a phase shifting circuit consisting of capacitor 115 and a resistor 116 connected between the emitter and collector of the transistor 105 and a phase shifting circuit consisting of a capacitor 117 and a resistor 118 connected between the emitter and collector of the transistor 106. Thus, the signals are changed to modified signals +L.DELTA..theta. and +R.DELTA..theta.. These signals +L.DELTA..theta. and +R.DELTA..theta. are respectively supplied to resistors 121 and 120 of the matrix circuit 93. A desired low frequency at which the phase is to be inverted can be selected by suitably selecting the values of capacitance of the capacitors 115 and 117 and the values of resistance of the resistors 116 and 118.

In the phase shifting circuits 91 and 92 having a construction as shown in FIG.6, the amount of phase shifting in the low frequency portion of the original signals +L and +R increases as the frequency decreases. Accordingly, in the above described circuits, the phase of the signal in all low frequencies is not uniformly shifted by 180.degree. from the phase of the original signal (phase inversion in a usual sense of the word). The modified signal which is inverted in phase in a desired low frequency portion according to the invention includes a modified signal obtained through the phase shifting circuits 91 and 92 which is changed in its phase in the desired low frequency portion from the phase of the original signal. Again, the polarities of the signals obtained from the phase shifting circuits 91 and 92 are different depending upon the construction of the circuits 91 and 92. The polarities have only to be ones which enables the difference signals to be obtained in the matrix circuit 93, and are not limited to the ones in the foregoing embodiment.

Input lines 125 and 126 of the variable crosstalk amount adjusting circuit 90 may be connected to points 127 and 128 on the emitters of the transistors 105 and 106 of the phase shifting circuits 91 and 92.

In the matrix circuit 93, a variable resistor 123 is connected between resistors 119 and 120. A variable resistor 124 which is linked with the variable resistor 123 is connected between resistors 121 and 122. Due to the resistors 119 and 120 and the variable resistor 123, the signal -L from the collector of the transistor 105 and the signal +R.DELTA..theta. from the emitter of the transistor 106 are mixed together, and a resultant difference signal is output from the slider of the variable resistor 123 and supplied to a variable resistor 129. Due also to the resistors 121 and 122 and the variable resistor 124, the signal -R from the collector of the transistor 106 and the signal +L.DELTA..theta. from the emitter of the transistor 105 are mixed together, and a resultant difference signal is output from the slider of the variable resistor 124 and supplied to a variable resistor 130 which is linked with the variable resistor 129.

The signals -L and +R.DELTA..theta. and the signals -R and +L.DELTA..theta. are respectively mixed with each other into difference signals [-(L-R.DELTA..theta.)] and [-(R-L.DELTA..theta.)]. It is to be noted here that the modified signals R.DELTA..theta. and L.DELTA..theta. which are deducted from the signal L and R have been obtained, as previously described, by inverting (changing) the phases of the signals L and R in low frequency portion. Accordingly, the low frequency portions in the difference signals [- (L-R.DELTA. 1/4)] and [- (R- L.DELTA..theta.)] are developed as sum signals [(the low frequency portion of the signal L) + (the low frequency portion of the signal R) ] and [(the low frequency portion of the signal R) + (the low frequency portion of the signal L) ] . The remaining portions of the modified signals R .DELTA..theta. and L .DELTA..theta. which have not been inverted (changed) in phase become difference signals which are of a composition similar to a simple difference signal. Accordingly, the difference signals [-(L -R .DELTA..theta.)] and [-(R -L .DELTA..theta.)] include a sufficient amount of low frequency component, so that an artificial stereophonic reproduced sound field having a vivid feeling of reality can be obtained when these signals are reproduced and sounded from the rear speakers.

By adjusting the linked variable resistors 123 and 124, the level ratio of the respective channel signal can be shifted from 1 : 1. Namely, if the respective slider of the variable resistors 123 and 124 are at their middle positions, a mixing ratio of the signal -L and the signal +R .DELTA..theta. and that of the signal -R and the signal +L .DELTA..theta. are respectively 1 : 1. If however, the sliders are at positions which are shifted from the middle positions, the mixing ratio of the signal -L and the signal +R .DELTA..theta. and that of the signal -R and the signal +L .DELTA..theta. are shifted from the ratio of 1 : 1. The output difference signals of the matrix circuit 93 in which the mixing ratio is shifted from 1 : 1 are expressed by [-(L'-R'.DELTA..theta.)] and [-(R'-L'.DELTA..theta.)]. By making the mixing ratio variable in the foregoing manner, a portion of signal component which might have been lost in a difference signal obtained by mixing the signals without changing the signal level, i.e., at a mixing ratio of 1 : 1 is retained. Consequently, the tone quality of the difference signal itself is improved and the stability of the whole reproduced sound field is increased. Thus, a reproduced sound field which causes no unpleasantness or feeling of disharmony to the listener can be created.

The output difference signals [-(L'-R'.DELTA..theta.)] and [-(R'-L'.DELTA..theta.)] of the matrix circuit 93 are adjusted in their level by the variable resistors 129 and 130 and then supplied to the bases of transistors 131 and 132 of the channel signal phase shifting circuits 94 and 95. A resistor 133 and a capacitor 134 are connected between the collector and the emitter of the transistor 131. A resistor 135 and a capacitor 136 are connected between the collector and the emitter of the transistor 132. Outputs are obtained from a connecting point of the resistor 133 and the capacitor 134 and a connecting point of the resistor 135 and the capacitor 136.

One embodiment of frequency-phase shift characteristic between the two channel signals in case the channel signal phase shifting circuits 94 and 95 are used is shown by curves I and II in FIG. 7. According to the characteristic curves in this embodiment, the phase difference between the two channel signals is small both in low and high sound frequencies. Modifications may be made, however, by, for example, making the phase difference between the two channel signals uniform over the whole sound frequency range or making the phase difference between the two channel signals small in a low frequency portion and uniformly large in other frequency portion.

The difference signals [-(L'-R'.DELTA..theta.).DELTA..phi..sub.1 ] and [-(R'-L'.DELTA..theta.).DELTA..phi..sub.2 ] which are given the phase difference within 90.degree. between the two channel signals in the channel signal phase shifting circuits 94 and 95 are respectively supplied to the bases of the transistors 137 and 138 of the amplifying and frequency circuits characteristic compensation circuits 96 and 97. In this case, the phase difference between the angles .phi..sub.1 and .phi..sub.2 should be held at 90.degree. at the maximum. As described above, the phases of the signal compents in opposite phase which are included relatively abundantly in both of the difference signals are shifted from the opposite phase relation in the channel signal phase shifting circuits 94 and 95. Thus, the sound of the reproduced sound field due to the rear signals are oriented to some extent, and the sound of the indirect sound component reproduced from the rear signals are made closely natural, having a feeling of definiteness and expansion.

The amplifier and frequency characteristic compensation circuits 96 and 97 consist respectively of amplifier circuits comprising transistors 137 and 138 and CR type tone quality adjusting circuits of a known construction comprising resistors 139 to 142, capacitors 147 and 148, a switch 151, resistors 143 and 146, capacitors 149 and 150 and a switch 152. Signals which have been amplified to the required level and compensated in their frequency characteristics in view of high frequency components, distortions, noise components etc. in the circuits 96 and 97 are reproduced and sounded as an indirect sound from the rear speakers 29 and 30. Thus, an excellent artificial four channel stereophonic reproduced sound field is created.

An example of other concrete modified circuit of the phase shifting circuits 91 and 92, the matrix circuits 93 and the channel signal phase shifting circuits 94 and 95 described with reference to FIG. 6 is shown in FIG. 8. In FIGS. 6 and 8, the same component parts are designated by the same reference numerals and the description thereof will be omitted. In FIG. 8, the signal -L from the collector of a transistor 105 and the signal +R .DELTA..theta. from the emitter of a transistor 106 are respectively mixed by resistors 160 and 162. The signal -R from the collector of the transistor 106 and the signal +L .DELTA..theta. from the emitter of the transistor 105 are respectively mixed and matrixed by resistors 163 and 161.

In a variable crosstalk amount adjusting circuit 90 for front left and front right channel signals, crosstalking is made in phase, for example, (0.92L + 0.38R) or (0.92R + 0.38L). Whereas, in phase shifting circuits 91 and 92 and a matrix circuit for rear left and rear right channel signals, crosstalking is made in opposite phase, for example, (0.92L - 0.38R) or (0.92R- 0.38L). In this case, the crosstalk in opposite phase is made with respect to 32Hz and over. In the difference signal of the two channel signals L and R, low frequency signals which are almost monaural signals in frequencies below 30 Hz leave a small portion of different waveform when they are cancelled by each other. This residual waveform produces distortions and unclearness in sound. In order to prevent occurrence of this residual waveform, in-phase crosstalk is made in frequencies below 30 Hz which have no influence on the stereophonic effect whatsoever.

The difference signals which have been crosstalked in opposite phase are supplied to the gates of field effect transistors (FET) 164 and 165. If impedances of resistors 133 and 135 connected to the drains of the FET's 164 and 165 are smaller than impedances of capacitors 134 and 136 connected to the sources thereof, the signals flow from the drains. If the impedances of the resistors 133 and 135 become greater than the impedances of the capacitors 134 and 136, the signals flow from the sources. On the other hand, the drain is in opposite phase and the source is in phase with the gate. Consequently, phase characteristic is 0.degree. in low frequencies and approaches -180.degree. as frequency increases, as shown in FIG. 9. The frequency at which the impedance of the resistor and that of the capacitor coincide with each other is the shift frequency, which becomes -90.degree..

In the present embodiment, the values of resistance of the resistors 133 and 135 are respectively 8.2 K.OMEGA. and 6.8 K.OMEGA. whereas the values of capacitance of the capacitors 134 and 136 are respectively 0.22 .mu.F and 0.04 .mu.F. The frequency f.sub.1 at which the phase lags by 90.degree. in respect of the channel system of the FET 164 is

f.sub.1 = 1/2.pi. .times. 8.2 .times. 10.sup.3 .times. 0.22 .times. 10.sup..sup.-6) .apprxeq. 88 Hz

The frequency f.sub.2 regarding the channel system of the FET 165 is

f.sub.2 = 1/2.pi. .times. 6.8 .times. 10.sup.3 .times. 0.047 .times. 10.sup.-.sup.6) .apprxeq. 500 Hz

FIG. 10 is an electrical circuit diagram of a fourth embodiment of an apparatus to which the system according to the invention is applicable. The left and right channel signals L and R from the input terminals 11 and 12 is applied through coupling circuits consisting of a resistor 172 and a capacitor 173, and a resistor 174 and a capacitor 175 to the bases of transistors 176 and 177 of amplifier circuits 170 and 171. Resistors 178, 179, 180 and 181 are respectively base biasing resistors for the transistors 176 and 177. Resistors 182 and 183 are respectively collector resistor and emitter resistor of the transistor 176. Resistors 184 and 185 are collector resistor and emitter resistor of the transistor 177.

A resistor 186 is connected between the emitter of the transistor 176 and the emitter of the transistor 177. The resistor 186 disposed between the two amplifier circuits 170 and 171 causes signals in the amplifier circuits to be crosstalked in opposite phase. Accordingly, difference signals which are in opposite phase with each other are obtained in respective amplifier circuits 170 and 171. Namely, the transistor 176 of the amplifier circuit 170 amplifies the input signal L applied to the base thereof and the signal R developed at the emitter of the transistor 177 and supplied through the resistor 186 to the emitter of the transistor 176, and outputs a difference signal [-(L -.alpha.R)] from its collector. The transistor 177 of the amplifier circuit 171 likewise amplifies the input signal R applied to the base thereof and the signal L developed at the emitter of the transistor 176 and supplied through the resistor 186 to the emitter of the transistor 177, and outputs a difference signal [-(R -.alpha.L)] from its collector.

The coefficient .alpha. in the above difference signals varies with the values of resistance of the emitter resistors 183 and 185 of the transistors 176 and 177 and the resistor 186. The crosstalk amount in opposite phase of the opposite channel signal to be given between the two amplifier circuits 170 and 171 can be set at a desired value by suitably selecting the values of resistance of each aforementioned resistor. If, for example, the coefficient .alpha. is selected at 0.5, an excellent result can be obtained.

The difference signal [-(L -.alpha.R)] developed at the collector of the transistor 176 passes through a capacitor 187 and, after being reinforced in a signal component in low frequencies by a low sound reinforcing circuit consisting of a capacitor 189 and a resistor 190 connected in series, is output from a slider 191a of a variable resistor 191. A difference signal [-(R -.alpha.L)] developed at the collector of the transistor 177 passes through a capacitor 188 and, after being reinforced in a signal component in low frequencies by a low sound reinforcing circuit consisting of a capacitor 192 and a resistor 193 connected in series, is output from a slider 194a of a variable resistor 194. The signals output from the sliders 191a and 194a of the variable resistors 191 and 194 are supplied through coupling circuits consisting of capacitors 199 and 200 and resistors 201 and 202 to the bases of transistors 197 and 198 of phase shifting circuits 195 and 196.

Inasmuch as the above described low sound reinforcing circuit is of a type in which the signal level in low sound frequencies is relatively reinforced by lowering the signal level in middle and high sound frequencies, an amplifier circuit is generally required. In the present embodiment, however, the circuits of the transistors 176 and 177 have gains, so that no particular amplifier circuit is required. Further, by constituting this low sound reinforcing circuit so that it has a characteristic that it can compensate difference in frequency-response characteristic between the two channels which takes place when phase shifting circuits 195 and 196 to be described later have frequency-response characteristics which are different from each other, an apparatus which has a balanced characteristic between the two channels can be constructed with relatively few component parts.

In the phase shifting circuits 195 and 196, resistors 203, 204 and resistors 205, 206 are respectively base biasing resistors of transistors 197 and 198. Resistors 207 and 209 are collector resistors and resistors 208 and 210 are emitter resistors of the transistors 197 and 198. A resistor 211 and a capacitor 212 are connected between the collector and emitter of the transistor 197. A resistor 213 and a capacitor 214 are connected between the collector and emitter of the transistor 198. The output of the phase shifting circuit 195 is obtained from a connecting point of the resistor 211 and the capacitor 212, and the output of the phase shifting circuit 196 is obtained from a connecting point of the resistor 213 and the capacitor 214.

The outputs of the phase shifting circuits 195 and 196 are signals [(L -.alpha.R) .DELTA..phi..sub.1 ] and [)R- .phi.L) .DELTA..phi..sub.2 ] which are signals in the neighborhood of an accoustically required frequency band and given a phase difference below 90.degree. from each other. Frequency-phase shifting characteristics of the phase shifting circuits 195 and 196 are shown in FIG. 11. The frequencies f.sub.1 and f.sub.2 shown in FIG. 11 are respectively expressed in the equations

f.sub.1 = 1/(2.pi. C.sub.212 R.sub.211) (1) f.sub.2 = 1/(2.pi. C.sub.214 (2) ub.213)

where C.sub.212 and C.sub.214 are values of capacitance of the capacitors 212 and 214, and R.sub.211 and R.sub.213 are values of resistance of the resistors 211 and 213.

The output signals [(L -.alpha.R) .DELTA..phi..sub.1 ] and [(R -.alpha.L) .DELTA..phi..sub.2 ] supplied from the phase shifting circuits 195 and 196 to resistors 217 and 218 through capacitors 215 and 216 are obtained from output terminals 219 and 220. The phase difference in the neighborhood of the accoustically required frequency band is .phi..sub.1 -.phi..sub.2 .apprxeq. 90.degree.. Due to the phase difference of about 90.degree. given to the two indirect rear signals in frequencies below several KHz by employment of the phase shifting circuits 195 and 196, a right side reverberating sound is heard from the right side and a left side reverberating sound is heard from the left side of the reproduced sound field. Accordingly, the unnatural feeling due to lack in the sense of orientation which is likely to occur when the two channel signals are composed in opposite phase with each other is prevented.

Nextly, one example of constants of each element used in the circuit shown in FIG. 10 is shown below.

Resistor 172, 174 470 K .OMEGA. 178, 180 " 179, 180 270 K .OMEGA. 182, 184 5.6 K .OMEGA. 183,185 " 186 5.6 K .OMEGA. 190, 193 8.2 K .OMEGA. 207, 210 4.7 K .OMEGA. 212 0.22 .mu.F 214 0.047 .mu.F 215, 216 0.68 .mu.F/25V

the frequencies f.sub.1 and f.sub.2 calculated on the basis of the foregoing constants are given as

f.sub.1 = 1(2.pi. .times. 0.022 .times. 10.sup.-.sup.6 .times. 8.2 .times. 10.sup.3) .apprxeq. 88 Hz

and

f.sub.2 = 1/(2.pi. .times. 0.047 .times. 10.sup.-.sup.6 .times. 6.8 .times. 10.sup.3) .apprxeq. 500 Hz

Accordingly, the sound from the left rear speaker has a phase difference of 90.degree. at 88 Hz relative to the sound from the left front speaker. The sound from the right rear speaker has a phase difference of 90.degree. at 500 Hz relative to the sound from the right front speaker. Again, the sounds from the left and right rear speakers have a phase difference of 90.degree. over a wide frequency range with a center frequency of 300 Hz.

In the meanwhile, the input signals L and R from the input terminals 11 and 12 are applied through resistors 221 and 222 and capacitors 223 and 224 to the bases of transistors 225 and 226 as front signals. The collector output of the transistor 225 is obtained from an output terminal 229 through a capacitor 227 and a resistor 228. The collector output of the transistor 226 is obtained from an output terminal 232 through a capacitor 230 and a resistor 231. The resistors 228 and 231 are connected by a resistor 233. Consequently, the outputs of the transistors 225 and 226 are crosstalked in phase, and signals (L + .DELTA.R) and (R + .DELTA.L) are respectively obtained from terminals 229 and 232.

While the invention has been described with respect to the specific embodiments, various modifications and variations thereof will be apparent to those skilled in the art without departing from the scope of the invention which is set forth in the appended claims.

Claims

What we claim is:

1. A multidimensional stereophonic reproducing system comprising a first terminal for receiving a first sound signal; a second terminal for receiving a second sound signal; first phase shifting circuit means responsive to said first sound signal coming through said first terminal for phase-shifting said first sound signal and producing a first modified signal, second phase shifting circuit means responsive to said second sound signal coming through said second terminal for phase-shifting said second sound signal and producing a second modified signal, each of said first and second phase shifting circuits being operative to cause a phase reversal of low frequency components of an applied input signal and substantially no phase change of high frequency components of an applied input signal, the phase shifting varying in dependence on the frequency of the applied input signal; matrix circuit means responsive to said first and second sound signals and the first and second modified signals for producing respectively a first difference signal and a second difference signal, said first difference signal being a difference signal between said first sound signal and said second modified signal, said second difference signal being a difference signal between said second sound signal and said first modified signal; third phase shifting circuit means responsive to the first and second difference signals for producing respectively a third sound signal and a fourth sound signal, either of which are phase-shifted from the first and second difference signals respectively such that the phase difference between the third and fourth sound signals is within ninety degrees; first amplifier means for amplifying said first sound signal to be radiated from the front left side with respect to a listener; second amplifier means for amplifying said second sound signal to be radiated from the front right side with respect to the listener; third amplifier means for amplifying said third sound signal to be radiated from the rear left side with respect to the listener; and fourth amplifier means for amplifying said fourth sound signal to be radiated from the rear right side with respect to the listener.

2. The reproducing system as defined in claim 1 wherein said matrix circuit means comprises variable resistor means for simultaneously changing level ratios of said first sound signal to said second modified signal and of said second sound signal to said first modified signal so that said first difference signal is a difference signal between the level-changed first sound signal and the level-changed second modified signal and said second difference signal is a difference signal between the level-changed second sound signal and the level-changed first modified signal.

3. The reproducing system as defined in claim 1 wherein said matrix circuit means produces said first difference signal by crosstalking in opposite phase, through at least one resistor having a predetermined resistance value, said first sound signal and said second modified signal, and also through at least one other resistor having the predetermined resistance value, said second sound signal and said first modified signal.

4. The reproducing system as defined in claim 3 further comprising means for crosstalking the first and second sound signals in phase between each other and producing the first sound signal crosstalked in a predetermined amount with the second sound signal and the second sound signal crosstalked in the predetermined amount with the first sound signal, said first amplifier means for amplifying the first sound signal crosstalked with the second sound signal, said second amplifier means for amplifying the second sound signal crosstalked with the first sound signal.

5. The reproducing system as defined in claim 1 wherein said first sound signal is a left channel signal in two channel stereophonic signals and said second sound signal is a right channel signal in the two channel stereophonic signals.

6. The reproducing system as defined in claim 1 wherein the first and second sound signals are respectively a left channel signal L and a right channel signal R in two channel stereophonic signals and said matrix circuit means crosstalks the left and right channel signals in opposite phase between each other with a predetermined amount of crosstalk .DELTA. and produces the first difference signal (L - .DELTA.R) and the second difference signal (R -.DELTA.L).

7. The reproducing system as defined in claim 6 further comprising means for crosstalking the left and right channel signals in phase between each other with the predetermined amount of crosstalk .DELTA. and producing signals (L + .DELTA.R) and (R + .DELTA.L), said first amplifier means for amplifying the signal (L + .DELTA.R), said second amplifier means for amplifying the signal (R + .DELTA.L).

8. The reproducing system as defined in claim 1 further comprising a first speaker disposed in the front left side in respect to the listener for transducing the output signal of said first amplifier means into a first sound, a second speaker disposed in the front right side in respect to the listener for transducing the output signal of said second amplifier means into a second sound, a third speaker disposed in the rear left side in respect to the listener for transducing the output signal of said third amplifier means into a third sound, and a fourth speaker disposed in the rear right side in respect to the listener for transducing the output signal of said fourth amplifier means into a fourth sound.